1. Field of the Invention
The present invention relates to an optical disk reproducing apparatus, and more particularly, to an optical disk reproducing apparatus able to compensate for tracking error, and a method of compensating for the tracking error.
2. Description of the Related Art
An optical disk reproducing apparatus can be a compact disk (CD) player, a digital video disk (DVD) player, a compact disk rewritable (CD-RW) drive, a CD-ROM drive, or a DVD-ROM drive. In general, in the optical disk reproducing apparatus, a laser beam is irradiated on tracks on the disk, reflected light therefrom is picked up by a pickup unit, and audio or video information contained therein is read from the picked-up reflected light. There are spiral tracks on the disk. The coded audio or video information is recorded as pits formed along the tracks. The coded information is read by the laser, provided to the pickup unit tracing the spiral tracks. If the disk tracks are not completely concentric or if a central shaft of a spindle motor for rotating the disk is shaken, a tracking error, that is, an eccentricity error, occurs. If the tracking error deviates from an error tolerance of the optical disk reproducing apparatus, the laser beam cannot accurately trace the tracks, and it may stray away from the tracks.
FIG. 1 is a block diagram illustrating an example of a conventional optical disk reproducing apparatus. The conventional optical disk reproducing apparatus comprises a spindle motor, which rotates an optical disk, a pickup unit 10, a driver 20, an RF amplifier 30, a first adder 40, a servo 50, a second adder 60, a disk rotation detection unit 70, an eccentricity control unit 80, and an eccentricity compensation unit 90. As shown in FIG. 1, the conventional optical disk reproducing apparatus uses a feedforward compensation method to add a compensation signal measured from the tracking error signal TE to a TRD signal. In another method, the conventional optical disk reproducing apparatus compensates for the tracking error by measuring compensation signals from the tracking error signal TE and the output signal TRD of the servo 50 and adding the compensation signals to the respective signal. In the conventional methods of compensating for the tracking error, the tracking error signal TE or the output signal TRD of the servo 50 is extracted during one or tens of disk rotations, and the extracted signal is stored in a memory. The signal stored in the memory is added as a compensation signal to the tracking error signal TE or the output signal TRD of the servo 50 in accordance with the disk rotational frequency, thereby compensating for the tracking error. In the conventional methods, it is difficult to accurately compensate for the tracking error due to external noise or interference when signal is generated. In addition, since the compensation is carried out only by using the values stored in the memory, it is difficult to suitably compensate for the tracking errors due to other causes. In addition, in some of the conventional methods, a compensation signal measured based on one disk rotation is stored, and the tracking error is compensated by using a fixed sampling frequency. In this case, although the number of values stored as the tracking error compensation signal is constant, the number of samples of the tracking error compensation signal varies-depending on speed of the spindle motor. As a result, it is impossible to suitably compensate for the tracking error due to the speed change in a constant angular velocity (CAV) mode or the rotational frequency difference between the inner and outer circumferences in a constant linear velocity (CLV) mode. One conventional optical disk reproducing apparatus is described in International Patent Application No. PCT/JP2000/02562.
In another method described in a patent to OAK Technology, tracking error is compensated by storing a predetermined servo output at every disk rotation, and adding a tracking error signal stored in a memory to the servo output. In this method, a disk rotation detection signal FG corresponding to one rotation of the disk is generated from the spindle motor and used for a sampling frequency in order to store the tracking error signal. For this reason, even if a rotational frequency difference in a CLV mode or the speed change in a CAV mode occurs, the number of samples is constant, and thus, the position of the tracking error signal over the compensation signal is constant. In addition, when the frequency band of the tracking error signal varies depending on the disk speed, it is very difficult to compensate for the change of the servo gain. Accordingly, it is difficult to compensate for the continuous change of the gain. Therefore, at the time of the change of the gain, an unstable compensation section is generated, and suitable tracking error compensation is impossible in the CLV mode.
For example, FIG. 2 shows error compensation characteristics obtained by using the conventional optical reproducing apparatus. In FIG. 2, the upper and lower waveforms correspond to a tracking error signal TE and a servo output signal TRD, respectively. In FIG. 2, regions (a), (b), and (c) indicate a compensation portion at a 24× speed, a speed changing portion, and a re-compensation portion at a changed 48× speed, respectively. As shown in FIG. 2, in a case where compensation is carried out in a lower speed and then the speed is increased, the frequency of the tracking error signal TE is increased in accordance with the increases in the speed. In turn, the increase in the frequency results in the decrease in the servo gain, so that the tracking error may increase. Therefore, it is necessary to additionally compensate for the gain or phase of the servo in accordance with the speed change. In particular, in the region (b), the increase in the tracking error causes the system to be unstable. In addition, if the spindle motor cannot generate at least 24 pulses for one disk rotation, the resolution deteriorates, so that it may severely affect the tracking error compensation performance.
That is, since the conventional optical reproducing apparatuses store tracking error signals in one or several disk rotations and use the stored tracking error signals as a compensation signal, it is difficult to accurately estimate and compensate for the tracking error. Since the conventional apparatuses use a fixed sampling frequency, it is difficult to cope with the speed change and to perform a suitable compensation. In addition, when the tracking error component is extracted at every edge of the disk rotation detection signal FG, since the resolution is determined depending on the number of the pulses output from the spindle motor, it is difficult to cope with the change of the frequency band of the tracking error signal. In addition, even though the tracking error component is extracted at every edge of the disk rotation detection signal FG, the gain of the servo needs to be newly set in accordance with the change of the tracking error signal frequency.